What is interesting, is the fact that PalmOS, Symbian and now WebOS all stopped existing in 2011.

PalmOS had a very good start and a lot of applications, but they did stop development when they had a very big market share. They missed the jump into multitasking...

Symbian, was a very promising system, but always had multiple GUI systems. So if you did develop a application for a Nokia Symbian, it did not run on a SE Symbian system.
When apple did then publish it's first (very limited) iPhone, Nokia and SE got hysteric and did mess up things. SE left the Symbian community and Nokia wanted to make it open source, with a single GUI system. But that step did take 1-2 years before even developers could start working on it...

I think still think that symbian has many advantages over iOS/Android, but due to such a weird GUI and horrible mobile data charges in the past, had no chance to survive.
And the Nokia company did a horrible job in press communication, so all developers started looking for other systems. (If you look at Nokia and the current "work" on MeeGo you see many paralels :( )

I'm not sure Nokia will be a big player in the future, this realy depends on what will happen with WM7.

iOS, yes, good marketing, sell a phone which does not even provide MMS, video conferences etc. But a cheap data plan, that did made the trick.
Personally I can't identify myself with iXXX products, but a valuable product, but too much in control of apple.

So we are reaching Android,
when it was becoming clear that Symbian will have not future (About 1 year after the launch of the P1i) I looked out for alternatives, there where only iOS and Android (Ok, Android only promising at the horizont at that time)
After two weeks of testing a iPhone 3 I returned to my P1i and waited for Android 2.x
The HTC device then came with a nice data plan, and it was love to the device. At first touching the screen was a bit strange, but now it works. (But I still miss the keyboard sometimes)
The main advantage of Android is, that I can write and install a application, without having to ask steve if he shows me his grace and allows my application.
Also I don't think that one phone model covers the need of all people of this world.

The main drawback of Android is currently the various versions and the (missing) updates.
Here Google has to make sure that the users get the updates. I had to wait many month for getting Gingerbread on my phone and even that was unsure.
In that area, the iPhone is better situated, of course, only one brand and 2 current phone models to support.

So what the future will bring us?

More iPhone/iOS devices, but only a few at a time

Even much more Android devices

WindowsMobile devices... there will be such devices but I'm not certain that they will have a big market share. MS has enough money to push WM in the next years, but is that enough ?

Blackberry, no, I don't think they have a big future with their own OS

Other OS ? No, what for ?

iOS covers the needs of the "All-in-one" liking users
Android covers the needs of all liking the diversity of the world
WindowsMobile if you still think it works best together with your desktop
There just is no need for yet another mobile OS, and if you see how much the different competitors fight against each other with patetents, then a startup just has no place (And no finance) in these ridiculous patent fights.

What do you think, has another mobile OS chances to reach a considerable market share ?